1. Field of the Invention
The present invention relates generally to apparatus for measuring the dew point temperature of a gaseous atmosphere for establishing its relative humidity, and in particular to apparatus for measuring the dew point temperature of hot, dirty air of the kind commonly found in industrial environments such as drying systems.
2. Prior Art
Industrial process drying is typically accomplished by circulating hot, dry air over or through a wet product. When the air becomes saturated with moisture, it must be replaced with fresh air. Large quantities of energy are used in heating this replacement air, much of which energy will be wasted if a given quantity of air is replaced before its full drying capacity has been used. On the other hand, the drying process will become inefficient if the air is allowed to remain in the system after it has become saturated with moisture. To achieve optimum efficiency, therefore, it is necessary to use a system controller that can cause the air in the dryer to be replaced at the proper time, and this controller must in turn be provided with a sensor to measure the relative humidity of the air in the dryer. In this regard, "humidity" is the amount of water vapor present in a given quantity of air. If the amount of water vapor present is less than the maximum possible at a given temperature, the amount is expressed in terms of relative humidity. "Relative humidity" is the actual mass of moisture in a given quantity of air divided by the mass of moisture that the same volume and temperature of air would contain if it were completely saturated.
The "dew point" is the temperature at which air with a given moisture content becomes saturated. This measurement is accomplished by artificially lowering the temperature of a surface and then noting the temperature at which moisture first condenses. Dew point temperatures can be converted directly to relative humidity measurements by reference to appropriate charts or tables. Although the foregoing discussion relates to water and air, it is equally applicable to the condensation of other liquids from other gases.
A dew point sensor for use in industrial environments should be able to operate with an accuracy of 5% or better in an atmosphere having a temperature between 200 and 300 degrees Fahrenheit and a relative humidity between 50% and 100% The sensor must be unaffected by the contaminants that are often present in industrial drying operations.
Many types of dew point sensors are known to the art. They indicate the relative humidity of an atmosphere indirectly by directly measuring the dew point temperature, as previously discussed.
Some dew point sensors are capacitive and resistive devices and employ a moisture-sensitive material to sense condensation. The moisture-sensitive material is formed into a condensation surface on the dew point sensor, and heat is slowly removed from the sensor until condensation begins. The moisture-sensitive material reacts to the presence of the moisture of condensation, for example by changing resistance. The temperature of the sensor is continuously monitored, and when this change of resistance is detected the temperature is recorded. From this temperature, the relative humidity can be determined by reference to the tables mentioned.
Another prior art form of dew point sensor is a condensing mirror type provided with a reflective surface. When the temperature of the reflective surface is reduced to the dew point temperature, condensation takes place The moisture of condensation is detected either by visual inspection or by the use of a photocell or other light-sensitive device that responds to the change in reflectivity caused by the presence of moisture
Unfortunately, none of the dew point measuring instruments just described is able adequately to satisfy the requirements for use in industrial processes. The high temperatures that occur in industrial processes damage the components used in many sensors, and the contaminants often found in such processes alter the absorption properties of many moisture-sensitive sensors and cloud the reflecting surfaces of reflector type sensors.
Various other methods of achieving acceptable accuracy under the conditions of industrial processes have been attempted. One such device is disclosed in U.S. Pat. No. 2,680,371 issued to Donath on June 8, 1954. The Donath apparatus takes advantage of the fact that water gives up heat when it changes from its gaseous to its liquid state. This effect is the converse of the well-known cooling effect experienced when water evaporates. Just as water chills the surface from which it evaporates, so it warms the surface upon which it condenses. The heat given off by condensing water is called the "heat of condensation", and the presence of this heat is indirectly detected by the Donath apparatus.
The Donath device is comprised of a reflecting condensation element; means to measure the temperature of the element; means to slowly reduce the temperature of the element by applying a coolant to its back side; and means to expose the front side of the element to the gaseous mixture to be tested. As the element cools past the dew point of the mixture of gases under test, condensation takes place. The temperature of the element rises slightly as the element absorbs the heat of condensation, and this rise in temperature is used to indicate the occurrence of condensation
The principal drawback to the Donath apparatus is that it does not actually detect the heat of condensation. Rather, it measures the temperature of the sensing element. Although the heat of condensation will cause the temperature of the sensing element to rise, there is a finite time delay between the occurrence of condensation and the diffusion of the heat of condensation throughout the element. Since the temperature of the element rises only in response to the latter event, the accuracy of the Donath apparatus is inherently limited.
The apparatus disclosed in U.S. Pat. No. 2,904,95, issued to Obermaier on Dec. 10, 1953, seeks to overcome the drawbacks of the Donath apparatus by providing a split sensing element. One part of the element is insulated from moisture such that its temperature is unaffected by the occurrence of condensation, and the difference in temperature between the insulated and the uninsulated parts is then compared. Although this arrangement is an improvement over the Donath apparatus, it still suffers from the inherent limitation that what is actually measured is the change in the temperature of the element rather than the heat of condensation.
U.S. Pat. No. 3,396,574, issued to Hanlein, et al., on July 9, 1965, also discloses apparatus for measuring the increase in temperature of a sensing element that results from the heat of condensation. The Hanlein apparatus employs a Peltier device to accomplish slow cooling of the sensing element, and it uses two thermocouples to detect the increase in the temperature of the sensing element that occurs after the heat of condensation has diffused through the sensing element. Like the Donath and Obermaier inventions, the Hanlein invention uses the event of the increase in temperature of the sensing element to approximate the event of the transfer of the heat of condensation to the element. The Hanlein device is therefore subject to the same limitations of accuracy.
It will be apparent from the foregoing that there is a need for a dew point measuring apparatus that can function in the hostile environment commonly present in industrial systems and that is not subject to the errors inherent in sensing devices known to the art. The present invention satisfies this need.